drm/virtio: rename sync_seq and last_seq

To be clearer about our intentions to associate sequence numbers
and fence IDs, let's rename these variables.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201119010809.528-5-gurchetansingh@chromium.org


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>